Interest in bonded arrays of optical fibers has existed for some time. For example, optical fiber waveguide ribbons are of interest for the construction of multi-channel transmission cable. In a typical optical fiber planar array called a ribbon, a plurality of fiber waveguides, typically twelve, are held in spaced positions parallel to each other generally in a common outer jacket or sheathing. Also, an outer surface of each of the optical fibers may be provided with a layer of a colorant material.
Optical fiber ribbon provides a modular design which simplifies the construction, installation and maintenance of optical fiber cable by eliminating the need to handle individual fibers. For example, the splicing and the connecting of the individual optical fibers in a ribbon is accomplished by splicing and connecting the much larger ribbon, if the fiber positions therein can be precisely fixed and maintained. Accurate alignment of the individual fibers during fabrication of the ribbon has presented a problem in the past. Additionally, a ribbon structure which maintains accurate alignment of the individual fibers during handling and use of the ribbon has been somewhat difficult.
These problems have been overcome with an adhesive sandwich ribbon, commonly referred to as ASR. The ASR includes a planar array of optical fibers held between two tapes. Each tape is a laminate comprising a relatively soft inner layer which contacts the optical fibers and a relatively hard outer layer which provides mechanical protection for the array. Typically, optical fiber ribbons have been made by causing a parallel array of optical fibers to be held between two flat longitudinally extending polyester tapes with each tape having an adhesive layer on one side. Generally, longitudinal side portions of the tapes overhang the optical fibers. See U.S. Pat. No. 4,147,407 which issued on Apr. 3, 1979 in the names of B. R. Eichenbaum and W. B. Gardner, and U.S. Pat. No. 3,920,432, which issued on Nov. 18, 1975 in the name of P. W. Smith. Also, each of the optical fibers may be provided with a colorant material for purposes of identification.
Such an optical fiber ribbon structure is advantageous in that it is modular and mechanically rugged, is compact, is suitable for simultaneous mass splicing and is relatively easy to manufacture. Mass splicing may be accomplished by means of a positive and negative chip arrangement such as is shown in U.S. Pat. No. 3,864,018 issued in the name of C. M. Miller on Feb. 4, 1975.
Because tapes must be aligned, tensioned and juxtaposed with a moving array of optical fibers, the processing speed of a tape-type ribboning line is not at an optimum. Reduced line speeds and the tapes which are used to hold the optical fibers add significantly to the cost of the ribbon. Further, there must be no crossovers of the optical fibers in the array in order to ensure error-free splicing.
The art has recognized at least some of these shortcomings. As a result, departures from the typical tape-type ribbon structure have become available in the marketplace. In some commercially available ribbons, the optical fibers are embedded in a mass of curable material and in some instances there is a covering layer of a relatively hard plastic material which provides the ribbon with mechanical protection. Typically, those current offerings are relatively thick, perhaps on the order of 0.450 mm, which may tend to warp.
A variation of the last-described ribbon structure in one in which an array of optical fibers are secured together with an adhesive matrix material which is disposed with only a minimal amount of adhesive material between each two adjacent fibers. Each optical fiber is provided with a single UV curable coating material or with dual coating materials comprising a primary coating material and a secondary coating material. A material such as a well known UV curable material which has been used to provide a secondary coating on the optical fibers is caused to be disposed between each two adjacent fibers. The final configuration may be one in which the adhesive material between adjacent fibers has a meniscus-type shape.
Although this type structure results in material savings, it has at least several disadvantages. First, it is difficult to obtain a planar array of optical fibers with the foregoing arrangement for bonding together the fibers. Also, because of the meniscus-type configuration of the adhesive material between adjacent optical fibers, optical fibers within adjacent arrays in a stack of ribbons tend to nest together. As a result, the optical fibers in each ribbon are not free to move as the cable is handled or during temperature swings. The use of a minimum amount of bonding material such as occurs in the meniscus structure ribbon also requires strong interfiber bonding with relatively high modulus material to prevent the ribbon from coming apart during cabling. The strong interfiber mechanical coupling can however restrict relaxation of fiber stresses and strains that are induced when the ribbon or cabled ribbon is fabricated and subsequently handled, installed or subjected to temperature swings. This is particularly true if the coating material directly adjacent to the fiber cladding has a relatively high modulus and the array bonding material also has a relatively high modulus. This may result in undesirable microbending with accompanying losses in performance or in the breakage of optical fibers or of an entire ribbon when a ribbon type cable is plowed into the ground.
The meniscus or any ribbon structure which includes an adhesive material that provide strong interfibers bonding also causes other problems. Attempts to mass strip the matrix material from all fibers in the array and to access single fibers from the array without breaking fibers or removing any colorant material from the accessed fiber and/or those adjacent to it may not be successful. Removal of colorant material from the fiber during access may destroy the color code identification of individual fibers making the ribbon less user friendly. Moreover, complex and expensive mechanical stripping tools may be required to remove the matrix material from the optical fibers.
Seemingly what the prior art lacks is an optical fiber ribbon structure which does not include tapes but one which overcomes problems associated with currently available, previously described non-tape ribbon. The sought-after structure should be one with sufficient interribbon and interfiber mobility to allow movement of the ribbons and the fibers during handling and installation without damaging the ribbon structure. Also, the sought-after structure should be mechanically rugged to withstand cabling operations and plowing of the cable into the ground during installation and should exhibit acceptable loss performance at temperatures as low as -40.degree. F. Notwithstanding these requirements, the ribbon should be compact in size, and be strippable with access to the individual optical fibers from either a ribbon end or from midspan without removal of coloring material from the fibers and without the need for complex tools.